The present invention relates to the recovery of olefin monomer from a nitrogen purged degassing step in the manufacture of polyolefin.
Following the polymerisation of olefin such as ethylene or propylene to polyethylene or polypropylene, the polymer contains residual olefin monomer. This is conventionally removed in a degassing step using nitrogen to strip the monomer from the polymer material. The nitrogen purge gas, now contaminated with olefin monomer and other reaction by products or monomer impurities such as ethane and isobutylene and methane, may be flared off or burned with fuel.
U.S. Pat. No. 4,498,910 (Benkmann) discloses a process in which ethylene is reacted to produce ethylene oxide and a purge gas is withdrawn that contains residual ethylene together with a large amount of methane and a smaller proportion of other gases which include oxygen and xe2x80x98inert gasesxe2x80x99, which may be argon and nitrogen. The ethylene is recovered by pressure swing adsorption (PSA) using a silica gel adsorbent. The reported recovery of ethylene is about 97%, but the recovery of ethane is only about 62%. There are significant levels of both ethane and ethylene in the residual gas from the PSA. Indeed, most of the ethane appears here and not in the ethylene rich stream.
U.S. Pat. No. 5,245,099 (Mitariten) discloses the recovery by PSA over silica gel of ethylene and heavier components from a hydrocarbon stream which also contains a minor amount of nitrogen. The light cut which is the residual gas from the PSA contains percent levels both of ethylene and of ethane and very large amounts of methane, which like the ethane concentrates there.
U.S. Pat. No. 4,849,537 (Ramachandran) discloses a process for making acrylonitrile from propane by dehydrogenation of the propane to propylene followed by aminoxidation using oxygen enriched air leading to a product stream containing the acrylonitrile from which acrylonitrile is removed as a liquid by quenching. This leaves an off-gas containing propylene which is recovered and recycled by subjecting the off-gas to PSA over silica gel. Once again, the residual gas from the PSA, or light stream, contains percent levels of hydrocarbons, with methane, ethane and ethylene concentrating there.
U.S. Pat. No. 4,781,896 (Wilmore) discloses a polyolefin manufacturing process in which olefin monomer is recovered by de-gassing polyolefin product and is recycled. The recovery is done without the supply of a purge gas and it is remarked that the use of a nitrogen purge stream makes it difficult and expensive to recover the olefin.
U.S. Pat. No. 5,769,927 (Gottschlich) recovers olefin from such a nitrogen purge gas for recycle to the polymerisation by a process of condensation, flash evaporation and membrane separation.
There is a need for an economic method for recovering olefin monomer from olefin polymer using a nitrogen purge gas and recovering the nitrogen without significant olefin content whilst maintaining a sufficient degree of recovery of the nitrogen.
The present invention now provides a process for the production of a polyolefin in which an olefin monomer is polymerised to produce said polyolefin and residual monomer is recovered, said process comprising subjecting a gas stream comprising said monomer and nitrogen to a PSA process in which said monomer is adsorbed on a periodically regenerated solid adsorbent to recover a purified gas stream containing said olefin and a nitrogen rich stream containing no less than 99% nitrogen and containing no less than 50% of the nitrogen content of the gas feed to the PSA process.
The problems of recovery of the olefin from the nitrogen remarked on in Wilmore are overcome by appropriate choice of adsorbent and process conditions and a nitrogen stream is obtained of sufficient purity for reuse or for use elsewhere in the plant, for instance for the pneumatic movement of solids. This is in contrast to the results obtained in other olefin recycling proposals such as those of Benkmann and Ramachandran. In Benkmann a significant amount of ethylene and most of the ethane find their way into the light stream, i.e. the residual gas from the PSA, which in addition to nitrogen contains substantial amounts of oxygen. In Ramachandran, the PSA waste stream contains percent levels of C2+ hydrocarbons (i.e. ethane, ethylene and heavier hydrocarbons). Furthermore, the ethane and ethylene, present only in small amounts in this system, actually concentrate in the light stream, i.e. the residual gas from the PSA.
The term xe2x80x9cPSAxe2x80x9d is used herein to include processes in which regeneration is accomplished essentially by pressure reduction rather than by heat addition and specifically includes VSA (vacuum swing adsorption) processes.
The adsorbent may be silica gel but is preferably alumina of suitable pore size and surface area, on the grounds of price and performance. Preferably then, the adsorbent is alumina having a surface area of no more than 900 m2/g, more preferably no more than 800 m2/g, most preferably no more than 600 m2/g. Where silica gel is used, the surface areas preferred are as given for alumina.
One option is to use a layered bed or a pair of beds in series containing an upstream (with respect to the on-line feed direction of gas flow) silica gel portion and a down-stream alumina portion. Adsorption of C2H4 is characterised by a higher rate of mass transfer or alumina. The linear driving force mass transfer coefficients that match measured breakthrough curves are 0.55 and 0.88 secxe2x88x921 for alumina and silica gel respectively. Placing the alumina at the product end of the adsorbent bed sharpens the C2 mass transfer zone.
For both silica gel and alumina it is preferred that the alumina has an average pore diameter of at least 1.7 nm, more preferably at least 2.0 nm.
Preferred particle sizes for the adsorbent are 0.25 to 4 mm.
The recovery of C2+ hydrocarbons from the monomer and nitrogen containing gas stream is preferably no less than 95%, more preferably no less than 99%. The ratio of ethylene in the monomer and nitrogen gas stream to the ethylene in the nitrogen rich stream is preferably at least 500 and the ratio of ethane in the monomer and nitrogen gas stream to the ethane in the nitrogen rich stream is preferably at least 50.
Preferably, at each stage of the PSA process, breakthrough of C2+hydrocarbons from the adsorbent is avoided.
The PSA process is preferably operated using four beds, generally as described in U.S. Pat. No. 3,430,418, but any number of beds may be employed as known in the art. Feed temperatures are suitably from 5 to 100xc2x0 C. at pressures of 1 to 20 bara. The beds may be regenerated at lower pressures from 0.1 to 2 bara.
The invention includes apparatus for use in the polymerisation of olefin comprising a polymerisation unit having an inlet for olefin monomer and an outlet for polymer connected to a polymer degassing unit having an inlet for nitrogen purge gas and an outlet for a nitrogen/monomer mixture connected to an olefin recovery unit comprising a compressor feeding said nitrogen/monomer mixture in a compressed state to an inlet of a liquid olefin separator having outlets for liquid olefin and for an olefin depleted nitrogen/olefin mixture, said outlet for liquid olefin being connected to recycle the olefin to the polymerisation unit, and a PSA gas separation unit connected to receive said olefin depleted nitrogen/olefin mixture and to produce therefrom a purified nitrogen stream containing no less than 99% nitrogen and a recovered olefin stream, said PSA unit being connected to return said recovered olefin stream to the inlet of said liquid olefin separator.